maintain traceability through standard cells.
However, the standard cell is not an indestructible item that will not
change its value over a period of time.
Therefore, periodic calibration
must be performed as with any other standard. Also, since the standard cell
is only calibrated every 360 days, we must make measurements between
calibrations to ensure that the cell's EMF value remains as given on the
calibration certificate. You, as an ACL technician, may be called upon to
check or even calibrate a standard cell. The measurements called for would
be made using the opposition measurement technique.
In this technique, two standard cells, (one being the standard and the
other being the test instrument), are compared against each other and only
the difference EMF is measured. A standard cell group of four cells will be
sent to the ACL as a reference standard.
Your own cell group, being the
test instrument, is compared against this transportable reference group.
When making the actual measurement, certain rules must be followed to the
letter. For example, the positive terminal of the left cell will always be
connected to the positive terminal of the measurement system, while the
positive terminal of the right cell is connected to the negative terminal of
the measurement system as shown in Figure 3-2. The two negative terminals
will be connected together; thus, the measurement will only measure the
difference between the two cells.
Opposition measurement connection
Giving one cell a value of 1.018122 and the other cell a value of 1.018116
volts, you can see that the measuring system will measure 6 microvolts.
Whether this reading would be positive or negative would be dependent upon the
position of the cells.
If the left cell is higher in amplitude the